1. Field of the Invention
The invention relates to a shock absorber, i.e. a so-called inertial shock absorber providing impact absorption during blanking.
2. Related Art
Shock absorbers of the hydraulic type where the absorbing effect is obtained by oil being pressed through a nozzle or a throttle valve are known in connection with mechanical and hydraulic presses. These shock absorbers are mounted parallel to the tool between the movable member of the press, i.e. the slide, and the stationary member of the press, i.e. the press table. When the cutting tool in the press closes and the punches of the tool initiate a punching out of articles from the material in a blanking operation, the press frame, which may be considered a very rigid spring, is stretched whereby energy is stored therein. When the article partially punched out no longer can bear the press force, the material breaks and the process force drops to zero. As a result, the heavily stretched press frame is relieved very quickly with a pulse-like impact. This dynamic effect on the press causes violent vibrations, and the hydraulic shock absorbers mounted parallel to the tool try to prevent this effect by producting a counteraction immediately upon breaking of the material in such a manner that the total load on the press can pass off uniformly as to time.
A hydraulic shock absorber produces, however, a counteraction proportional to the relative speed of the tool members and since the speed of these members is relatively slow in the moment of break through, no significant reaction force is produced by the hydraulic shock absorber in the moment of break through. An efficient counteraction is not produced by the hydraulic shock absorbers until the stretched press frame has accelerated the press and tool members to a sufficiently high speed over a certain length. This delayed reaction causes unavoidable force steps in the press frame and limits consequently the noise and vibration preventing effect of the hydraulic shock absorber.
Hydraulic shock absorbers are often situated on the press table or corresponding supports, whereafter the height of the absorbing pistons is adjusted in such a manner that the hydraulic shock absorbers are not activated until the slide abuts them immediately before the material breaks. Now the speed of the slide relative to the press table is relatively low so that the impact-like coupling of the hydraulic shock absorbers causes no problems of importance.
Hydraulic shock absorbers cannot be secured at both ends to the slide and the press table, respectively because the reaction force of the shock absorbers in the first part of the press stroke before the tool is activated is much too great and consequently involves an overwhelming loss of energy. For the same reason hydraulic absorbers are only employable in connection with tools involving a cutting on a level adjacent the bottom of the stroke, i.e. the bottom dead centre.
This loss of energy at a permanent coupling of hydraulic shock absorbers is due to the fact that the speed reached by the press members immediately after the breaking of the material is of the same magnitudc (only 2-3 times greater) as the maximum speed reached in an idle stroke without material in the tool. In other words, in order to provide a sufficient counteraction at the breaking of the material the shock absorber has an absorbing coefficient adapted to the size of the nozzle which implies that the reaction effect on the movement of the slide is much too great in the steps of the stroke where no absorbing effect is necessary. Furthermore, the effect of hydraulic shock absorbers when used on presses is limited because of the following conditions:
(a) A press is of an extremely rigid and dynamic structure, where the deflection typicaly only amounts to 1-2 mm at the max. press force. This deflection is too short for utilizing a shock absorber efficiently.
(b) A hydraulic shock absorber produces a reaction force proportional to the relative speed of the members, which means that when the break through occurs and the press force drops to zero, only a modest absorbing effect applies as the relative speed of the tool members is low. The acceleration of the tool members relative to an idle stroke is, however, typically 100 to 150 times greater as to the release of the
spring energy in the stretched press frame.
The absorbers possess no substantial counteraction until the above acceleration force has made the press and tool members reach a sufficiently high speed over a certain length (0.25-0.50 mm). At this moment a substantial portion of the stored energy has been converted into kinetic energy, and nothing but a partial lowering of the vibration level and consequently of the noise level occurs.
(c) Hydraulic shock absorbers cannot be provided with very small nozzles as an attempt at reducing he reaction time and in order to obtain an increased absorbing coefficient because the continued movement of the slide towards the bottom dead centre must be ensured without a too violcnt counteraction and consequently without loss of energy and press force.
Various initiatives have been made of increasing the pressure-producing speed in the hydraulic shock absorber and consequently of reducing the reaction time, which for instance has been described in:
DE-OS No. 25 12 822, where the absorbing coefficient is made proportional to the deflection of the press frame by means of a mechanical rod system,
DE-AS No. 26 53 714, where the oil pressure in the absorbing cylinder is controlled by an ultrafast electro-hydraulic servo valve,
DE-AS No. 27 48 145, where the absorbing coefficient is adjusted by means of a cone valve controlled by the oil pressure in the absorber.
All the above absorbers are characterised by a high complexity and a certain degree of specializing and limitation as to use.
The U.S. patent specifications Nos. 4,054,186 "Snubber Apparatus" and 4,350,232 "Mechanical Shock Arrestor" disclose shock absorbers converting a linear, telescopic movement into a rotation of an inertial mass connected to a frictional braking device. The U.S. patent specification No. 4,289,218 "Mechanical Motion-Snubbing Device" discloses a corresponding shock absorber converting a linear, telescopic movement into a rotation of an inertial mass and where the energy is absorbed by a collision between said inertial mass and one or more other inertial masses. These absorbers react against all accelerating movements in both axial directions and dissipate the energy continuously. These absorbers are constructed to protect atomic plant installations and pipelines against seismic vibrations, explosions and the like catastrophes, and are not suited for shock absorbing in a machine tool.